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What is 8" mean in a 8" Lathe Chuck?

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Plastic
Joined
Aug 11, 2022
Hi, my town wood house lathe came with a 3 jaw chuck and I'm planning to buy a 4 jaw. The OD is roughly 8" and I'm looking for a similar sized one.

As I was searching online for 4 jaws, I couldn't find the OD mentioned in those sites. I guess the 8" in a 8" chuck means that is the max dia of round stock it can hold and I'm assuming by reversing the jaws, the chuck can (at least in theory) hold stock larger than it's diameter?

So if my current OD is roughly 8", should I assume it's 10"?

Appreciate your insights on this.
 
You can reverse the jaws to hold bigger stuff, but with the jaws in or the jaws out ways you still have to think about the jaws sticking out too far to bump something when thinking about chuck size.
I really like 4 jaws but one has to still think about wobble on long parts. Sometimes one may hold a part short indicate-in and center drill that end. Then put the centered end out on that tail stock center.

On an old clunker lathe I used to tool bit scalp a little pre-center drill scalp in the part end.

If the part is to be used in a fast application one may need to think about balance, so the body as well as the bearing-fit ends should run pretty true.

Also, true to near center is important if the raw stock size has little stock size and may not clean up making size.
When making something like a precision bearinged shaft you want zero wobble one bearing fit to the other because that beats up the bearings and makes running vibrate.
 
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As Greg says the 8", or 200 mm if it's in metric size, refers to body diameter. Modern chucks generally have the reverse jaws marked to indicate maximum safe extension. On all the ones I've seen the mark pretty much sets maximum holding capacity quite close to the body diameter. Usually a bit over.

A fundamental safety factor as if the jaws are too far out there won't be enough engagement between jaw and scroll to ensure things are not only safely held but also that there is sufficient friction between jaws and scroll to ensure the chuck cannot self loosen.

Most important if you've got a clunk-clunk-clunk job on the go.

Economy range imports can be a bit skimpy here. The little 5C spigot 5" diameter Chinese one I have is totally unreasonably accurate for the price (had to get lucky some time) but could definitely do with another turn on the scroll.

As could a couple of the 8" D1-5 cam lock mount ones I auditioned for my P&W Model B before lucking into an excellent NOS Bison at around 75% discount from the official price. Being uncommon can make stuff hard too find but sometimes the dusty shelf at the back of the shop yields treasure.

Generally you go up a size from the three jaw when sourcing a four jaw.

So I have an 8" three jaw and 10" four jaw on the P&W.

Normal duty four jaw chuck jaws are pretty chunky to give plenty of holding power which comes at the expense of finesse when gripping lighter or more complex things.

I'm a fan of light duty four jaw chucks in the larger sizes as these use jaws pretty much same size those on the next size down normal duty chucks in a less massive body. The 8" Pratt Bernerd light duty four jaw I have for my Smart & Brown 1024 truly hits the sweet spot for my work. I have no intention of doing work that needs heavy duty chuck gripping power on machines I've paid for out of my own pocket!

Clive
 








 
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